The invention relates to a method of producing optical fibers for telecommunication purposes, the fibers thus obtained and the cables constructed thereof.
For producing such fibers U.S. Pat. No. 3,941,474 describes a method in which two glasses, which differ from one another in the cation oxides incorporated therein, are melted in a double crucible with coaxial nozzle openings at the bottoms of an inner and an outer crucible. The core glass of the fiber has the highest refractive index and the cladding glass the lowest. If no diffusion would occur a fiber having a stepped refractive index profile would be obtained. Depending on the temperature prevailing in the interface region, the time the glass remains in this region, the diffusion behavior of the cation components of the two glasses, and the contributions to the refractive indices, the stepped refractive index profile will diffuse out to a graded profile. Such a profile is desired because it decreases the pulse dispersion in multimode fibers. Calculations have proven that this profile must be approximately parabolic to furnish a minimal mode dispersion.
A drawback of this known method for obtaining a fiber profile is that the shape of the refractive index profile ultimately obtained after diffusion does not sufficiently resemble the desired parabolic shape. It is known that the shape of a profile obtained by diffusion with a diffusion coefficient which is independent of location is characterized by a concentration decay (a so called diffusion tail) which varies slowly with an increasing radius. It is indeed possible to somewhat correct the profile shape by varying the amount of the diffusion at the core-cladding interface but this cannot sufficiently eliminate the deviation from the parabolic shape of the profile, especially at extremities. The only possibility, for eliminating the deviation, seems to be in finding a different diffusion behavior in which the diffusion depends on location owing to the dependence on concentration and/or temperature, and, consequently, proceeds in the desired manner. The glasses used for producing fibers in accordance with the double crucible method consist of four or more components and must satisfy a plurality of physical requirements to enable a successful processing into fibers. Finding empirically the compositions which satisfy all requirements, including the desired diffusion behavior, is a time consuming method, the success of which cannot be definitely predicted.